Memory leak in eval()'d code

class definitions due to the existence of opaque references. I still don't understand *why*, then, the memory stays flat when the new class does not happen in eval(): ``` for ($i = 0; $i < 1000000; $i++) { $object = new class {}; if ($i % 1000 == 0) { echo memory_get_usage() . "\n"; } } ``` I checked get_class() for all instances returned, **it's always the same class name whether it's called from eval() or not**, so this code should only ever create a single class in memory, right? Why the leak with eval()?

Interesting. Can this behaviour be relied upon (is it documented?), or is this an implementation detail that may change at any time? Thank you, Ben

Thanks for the example. Now I understand the root cause. There is a similar bug report(https://bugs.php.net/bug.php?id=74938) for anonymous function. I thought it leaked for the same reason, but now I think I’m wrong. We don’t have to create new classes for them since they are all instances of Closure class, and it’s not possible to create opaque reference for functions. __FUNCTION__ returns “{closure}”. Could you take a look? Back to the anonymous class, I’m thinking if we can have any workaround for this issue, otherwise it will be a huge problem if someone wants to write applications running for a long time. It will be a memory leak that cannot be fixed in userland code unless he decides to drop anonymous class at all. Since most of the use cases should not have opaque references, can we add a flag in zval to indicate if an object has this kind of reference? The number of methods to create such opaque references should be limited. __CLASS__, get_class(), reflections, clone. Did I miss any other way? With the help of this flag, we are able to know if it’s safe to remove the class definition during destruction. Regards, CHU Zhaowei From: Nikita Popov <nikita.ppv@gmail.com> Sent: Saturday, June 29, 2019 3:58 PM To: CHU Zhaowei <me@jhdxr.com> Cc: Stanislav Malyshev <smalyshev@gmail.com>; Benjamin Morel <benjamin.morel@gmail.com>; PHP Internals <internals@lists.php.net> Subject: Re: [PHP-DEV] Memory leak in eval()'d code On Sat, Jun 29, 2019 at 9:07 AM CHU Zhaowei <me@jhdxr.com <mailto:me@jhdxr.com> > wrote: I think we missed the point here. Clousre, or anonymous class, should not be considered as normal class. We expect normal class existing once we declare it till end of script. However, for anonymous class, it's usually used within certain scope, so not only the instances, the class itself should be included in the GC as well. I guess the problem here is we didn't GC the space for definition of anonymous classes. Regards, CHU Zhaowei As Johannes already pointed out, we cannot garbage collect anonymous class definitions due to the existence of opaque references. A simple example of code that currently works: $obj = eval('return new class {}'); $class = get_class($obj); // create opaque reference unset($obj); // drop last direct reference $obj = new $class; In the end, an anonymous class is essentially a class with a random name and some inline construction sugar, but apart from that does not differ from ordinary classes. (I've also seen some calls to allow syntax like $class = class {}; that would make that even more obvious.) The situation here would be different if we had first-class classes and did not refer to classes by name. But as-is, I don't think garbage collecting anonymous classes is a possibility. Nikita